In an electronic device, a switching power supply is used for supplying power to a load. For example, a DC-DC converter that converts a direct-current voltage into a different direct-current voltage is used. Japanese Laid-Open Patent Publication No. 2010-051073 describes a comparator-type DC-DC converter that may respond at a high speed to a sudden change in a load.
FIG. 14 illustrates an example of a comparator-type DC-DC converter 4 according to a related alt. The DC-DC converter 4 includes a converter unit 5 and a control circuit 6. The converter unit 5 includes transistors T11 and T12, a coil L11, and a capacitor C11.
A comparator 80 provided in the control circuit 6 receives a feedback voltage VFB1, which is generated according to an output voltage Vo1, and a reference voltage VR11. In FIG. 14, the feedback voltage VFB1 is generated by adding an alternating-current component of the output voltage Vo1 supplied through a capacitor C12 coupled in parallel to a resistor R11, to a divided voltage Vn obtained by dividing the output voltage Vo1 by resistors R11 and R12. The comparator 80 compares the feedback voltage VFB1 with the reference voltage VR11, and generates an signal S11 having a level according to a result of the comparison. The output signal S11 is supplied to a set terminal S of an RS-flip-flop (RS-FF circuit) 81. An oscillator 82 generates a dock signal CLK of a constant frequency, and supplies the dock signal CLK to a reset terminal R of the RS-FF circuit 81.
The RS-FF circuit 81 shifts to a reset state in response to the clock signal CLK of an H level and generates an output signal S12 of an L level. In this case, a drive circuit 83 generates control signals DH and DL of an H level, and deactivates the transistor T11 and activates the transistor T12. At this time, a switch circuit SW11 is turned off by the output signal S12 of the L level, and a capacitor C13 is charged according to a current I11 supplied from a current source 84. Therefore, the reference voltage VR11 increases at a fixed slope (I11/C13) from a reference voltage VR0.
When the reference voltage VR11 becomes higher than the feedback voltage VFB1, the comparator 80 generates the output signal S11, in response to the output signal S11 of an H level, the RS-FF circuit 81 shifts to a set state, and generates the output signal S12 of an H level. In this case, the drive circuit 83 generates the control signals DH and DL of an L level, and activates the transistor T11 and deactivates the transistor T12.
In this manner, in the comparator-type DC-DC converter 4, the comparator 80 compares the feedback voltage VFB1 according to the output voltage Vo1 with the reference voltage VR11. Then, the transistor T11 is switched according to a result of the comparison. Therefore, the comparator-type DC-DC converter 4 may respond at a high speed to a sudden change in a load.
In recent years, in DC-DC converters, because of low cost and other reasons, a ceramic capacitor of a low equivalent series resistance ESR is employed in many cases for the capacitor C11 in the converter unit 5. However, when a value of the equivalent series resistance ESR of the ceramic capacitor is too low, phase margin may not be sufficiently secured. Therefore, in a high-frequency operation when a load suddenly changes, ringing sometimes occurs in the output voltage Vo1.